Cytogenetic analysis in several laboratories indicates that roughly half of all spontaneous abortuses in humans are caused by a trisomic condition, mainly for the autosomes, which resulted from non-disjunction in the germ line. In research aimed to establish induced mutation rates in laboratory animals, either after application of X-, gamma-rays, fast neutrons or chemicals, non-disjunction produced during gametogenesis should be one of the end-points for cytological scoring. Mutation induction as a basis for risk assessment has been carried out with the laboratory mouse mainly. Contrary to man, the mouse has a very low spontaneous non-disjunction rate. No sensitive cytological scoring system has yet been developed for the test of chromosome non-disjunction during meiosis in this mammal. The purpose of the research proposed is to test a number of chromosome systems in the mouse which either have high spontaneous non-disjunction rates or otherwise are expected to do so, the hypothesis being that systems which exhibit high spontaneous non-disjunction are sensitive for induced non-disjunction too. Non-disjunction will be first induced by spindle poisons (among which there are with social-medical importance) which aids in ranking the chromosome systems proposed for sensitivity. All systems are characterized by marker chromosomes, which can be recognized during the first and second meiotic division. The spindle poisons can be applied during prometaphase, metaphase, and anaphase of the first meiotic division. A second period to focus attention on is the period of chromosome synapsis during zygotene. Both male and female mice can be used for this. The results are obtained after cytological scoring of secondary meiocytes. Once a sensitive system is established, known and suspected chemical mutagens can be tested for their non-disjunction inducing capacity.